For the following compounds, write structural formulas and IUPAC names for all possible isomers having the number of double or triple bond as indicated:
(a) `C_(4)H_(8)`(one double bond)
(b) `C_(5)H_(8)` (one triple bond)

Answer

(a) (i) `CH_(2)=CH-CH_(2)-CH_(3)" "`But-1-ene
(ii) `CH_(3)-CH_(2)=CH-CH_(3)" "`But-2-ene
(iii) `{:(CH_(2)=C-CH_(3)),(" "|),(" "CH_(3)):}`" "`2-Methylpropene
(b) (i)`HC-=C-CH_(2)-CH_(2)-CH_(3)" "`Pent-1-yne
(ii) `CH_(3)-C-=C-CH_(2)-CH_(3)" "`Pent-2-yne
(iii) (iii) `{:(CH_(3)-CH-C-=CH),(" "|),(" "CH_(3)):}" "`3-Methylbut-1-yne

Explanation

(a) The following structural isomers are possible for C4H8 with one double bond :
`underset((I))(H_(2)overset(1)C=overset(2)CH-overset(3)CH_(2)-overset(4)CH_(3))`
`underset((II))(overset(1)CH_(3)-overset(2)CH=overset(3)CH-overset(4)CH_(3))`
`underset((III))(overset(1)CH_(2)=underset(CH_(3))underset(|)overset(2)C-overset(3)CH_(3))`
The IUPAC name of
Compound (I) is But-1-ene,
Compound (II) is But-2-ene, and
Compound (III) is 2-Methylprop-1-ene.
(b) The following structural isomers are possible for C5C8 with one triple bond:
`underset((I))(Hoverset(1)Cequivoverset(2)C-overset(3)CH_(2)-overset(4)CH_(2)-overset(5)CH_(3))`
`underset((II))(H_(3)overset(1)C-overset(2)Cequivoverset(3)C-overset(4)CH_(2)-overset(5)CH_(3))`
`underset((III))(H_(3)overset(4)C-underset(CH_(3))underset(|)overset(3)CH-overset(2)Cequivoverset(1)CH)`
The IUPAC name of
Compound (I) is Pent-1-yne,
Compound (II) is Pent-2-yne, and
Compound (III) is 3-Methylbut-1-ene.